Gas concentration measuring apparatus designed to minimize error component contained in output

ABSTRACT

A gas concentration measuring apparatus which has a gas sensor designed to measure, for example, the concentrations of O 2  and HO x  contained in exhaust emissions of an automotive engine is provided. The apparatus includes a signal processing circuit which converts a current signal outputted from the gas sensor as a function of the concentration of either of O 2  and NO x  into a voltage signal. The gas sensor and the signal processing circuit are connected electrically through a conductor. The conductor has a length which is determined as a function of a level of the current signal outputted from the gas sensor. The weaker the level of the current signal is, the shorter the length of the conductor. This minimizes addition of electrical noises to the current signal outputted from the gas sensor.

This is a division of our earlier commonly assigned application Ser. No.09/453,518 filed Dec. 3, 1999, now U.S. Pat. No. 6,547,955 B1.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates generally to a gas concentration measuringapparatus for measuring the concentration of gases which may be employedin an air-fuel ratio control system for automotive vehicles, and moreparticularly to a gas concentration measuring apparatus designed tominimize an error component contained in an output thereof.

2. Background Art

Recently, NOx sensors designed to measure the concentration of nitrogenoxide (NOx) contained in exhaust emissions of automotive engines areproposed and put into practical use.

As one of such NOx sensors, a gas sensor is known which is designed tomeasure the concentrations of NOx and O₂ contained in exhaust gasses ofthe engine simultaneously. This type of gas sensor includes a pump cellfor decomposing or ionizing oxygen molecules contained in exhaust gassesto measure the concentration of O₂ and a sensor cell for decomposing NOxin the oxygen-decomposed exhaust gasses to measure the concentration ofNOx. The measurement of the concentration of each of NOx and O₂ isachieved by applying a given voltage to a corresponding one of the pumpcell and the sensor cell to induce flow of current as a function of oneof the concentrations of NOx and O₂. The current is outputted from thegas sensor and converted into a voltage signal which is, in turn, usedin, for example, an engine control unit of the vehicle.

The above gas sensor, however, has the drawback in that the amount ofcurrent flowing through the cell sensor as a function of theconcentration of NOx is extremely small, so that it apt to interferewith electrical noises, resulting in a failure in measuring theconcentration of NOx accurately. Specifically, when the concentration ofNOx is within 0 to 2000 ppm, a current output from the sensor cell is aslittle as 5 to 10 μA. Therefore, in the case where the gas sensor isused in an engine control system of an automotive vehicle, signaloutputs from peripheral electrical devices are added to an output of thegas sensor as noises which will produce an error in measuring theconcentration of NOx.

SUMMARY OF THE INVENTION

It is therefore a principal object of the present invention to avoid thedisadvantages of the prior art.

It is another object of the present invention to provide a gasconcentration measuring apparatus designed to minimize an errorcomponent contained in an output of the apparatus.

According to one aspect of the invention, there is provided a gasconcentration measuring apparatus. The gas concentration measuringapparatus includes: (a) a gas concentration sensor outputting a signalas a function of concentration of a given component of gasses; (b) asignal processing circuit processing the signal outputted from the gasconcentration sensor to produce a voltage signal indicative of theconcentration of the given component of the gasses; and (c) a conductorelectrically connecting the gas concentration sensor and the signalprocessing circuit for transmission of the signal. The conductor has alength which is determined as a function of a level of the signaloutputted from the gas concentration sensor. The weaker the level of thesignal is, the shorter the length of the conductor.

In the preferred mode of the invention, a connector is provided whichconnects the gas concentration sensor with an external device. Theconnector has disposed therein the signal processing circuit.

An impedance measuring circuit is provided which measures the impedanceof a sensor element of the gas concentration sensor. The impedancemeasuring circuit is integrated in a single unit together with thesignal processing circuit.

A heater and a heater control circuit are provided. The heater heats upa sensor element of the gas concentration sensor. The heater controlcircuit controls a power supply to the heater. The heater controlcircuit is integrated in a single unit together with the signalprocessing circuit.

The gas concentration measuring apparatus may be mounted in a vehicle tomeasure, for example, the concentrations of O₂ and NOx contained inexhaust emissions of a combustion engine for use in an air-fuel ratiocontrol. The weaker the level of the signal is, the shorter a distancebetween the gas concentration sensor and the signal processing circuitfor minimizing addition of electrical noises produced by electricaldevices mounted in the vehicle to the signal outputted from the gasconcentration sensor.

The gas concentration sensor includes a first cell responsive toapplication of a voltage to discharge oxygen contained in the gassesoutside the gas concentration sensor, producing a first electric currentas a function of concentration of the discharged oxygen and a secondcell responsive to application of a voltage to produce a second electriccurrent as a function of concentration of a specified gas componentcontained in the gasses from which the oxygen is discharged by the firstcell.

The signal processing circuit has a function of compensating for aunit-to-unit variation in characteristic of the gas concentrationsensor.

The signal processing circuit corrects an output characteristic of thegas concentration sensor so as to agree with a desired one.

The impedance measuring circuit has a function of compensating for aunit-to-unit variation in characteristic of the gas concentrationsensor.

The impedance measuring circuit produces an impedance signal indicativeof the impedance of the sensor element of the gas concentration sensorand corrects the impedance signal so as to eliminate a variation in theimpedance signal caused by the unit-to-unit variation in characteristicof the gas concentration sensor.

The heater control circuit connects with the heater through a powersupply conductor for supplying the power to the heater. The heatercontrol circuit has a function of minimizing an error component causedby a resistance value of the power supply conductor.

The signal processing circuit, the impedance measuring circuit, and theheater control circuit are formed on a bare chip mounted on a ceramicsubstrate.

According to another aspect of the invention, there is provided a gasconcentration measuring apparatus which comprises: (a) a gasconcentration sensor outputting a signal as a function of concentrationof a given component of gasses; (b) a signal processing circuitprocessing the signal outputted from the gas concentration sensor toprovide a voltage signal indicative of the concentration of the givencomponent of the gasses; and (c) a connector having disposed therein thesignal processing circuit, the connector having a first end coupled tothe signal processing circuit and a second end providing electricalconnection with an external device to transmit the voltage signal to theexternal device.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given hereinbelow and from the accompanying drawings of thepreferred embodiments of the invention, which, however, should not betaken to limit the invention to the specific embodiments but are for thepurpose of explanation and understanding only.

In the drawings:

FIG. 1 is a block diagram which shows a gas concentration measuringapparatus according to the invention;

FIG. 2 is an illustration which shows structures of a gas concentrationsensor and a sensor control circuit;

FIG. 3 is a sectional view which shows an internal structure of a gasconcentration sensor;

FIGS. 4(a), 4(b), and 4(c) are sectional views which show a sequence ofgas measurement operations of a gas concentration sensor;

FIG. 5 is a graph which shows a relation between a pump cell currentproduced by a pump cell and a voltage applied to the pump cell;

FIG. 6 is a graph which shows a relation between a sensor cell currentflowing through a sensor cell and a voltage applied to the sensor cell;

FIG. 7 is a circuit diagram which shows structures of a sensor controlcircuit and a gas concentration sensor;

FIG. 8 is a flowchart of a program performed to control voltages appliedto a pump cell and a sensor cell of a gas concentration sensor;

FIG. 9 is a flowchart of a program performed to measure the impedance ofa sensor element of a gas concentration sensor;

FIG. 10 is a time chart which shows a relation among an output voltageVs, a terminal voltage Vc, and a terminal voltage Ve;

FIG. 11 is a perspective view which shows a gas concentration sensor anda connector in which a sensor control circuit and a heater controlcircuit are disposed;

FIG. 12 is a graph which shows a relation of levels of signal outputsfrom a cup-shaped A/F sensor, a laminated A/F sensor, and a NOx sensorto the length of an output line extending from each of the sensors;

FIG. 13 is a graph which shows a relation between an actual output of agas concentration sensor and a correct one; and

FIG. 14 is a block diagram which shows a gas concentration measuringapparatus in which output correction circuits are disposed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like numbers refer to like partsin several views, particularly to FIG. 1, there is shown a gasconcentration measuring apparatus according to the invention which isused with, as one example, an automotive engine control system designedto control the quantity of fuel injected into an internal combustionengine as a function of an output of the gas concentration measuringapparatus under feedback (F/B) control to bring the air-fuel (A/F) ratiointo agreement with a target value and to diagnose the deterioration ofa catalytic converter installed in an exhaust pipe of the engine.

The gas concentration measuring apparatus uses a composite gasconcentration sensor 100 capable of measuring concentrations of oxygen(O₂) and nitrogen oxide (NOx) contained in exhaust gasses of amulti-cylinder four-cycle engine simultaneously.

A fuel injector 12 is installed in an intake pipe 11 to supply the fuelto the engine 10. The gas concentration sensor 100 is installed in anexhaust pipe 13 and outputs sensor signals indicative of theconcentration of O₂ and NOx.

The gas concentration sensor 100 has, as shown in FIG. 3, a two-cellstructure designed to measure the concentrations of O₂ and NOx containedin exhaust gasses of the engine 10 simultaneously. The gas concentrationsensor 100 is made of a lamination of the pump cell 110, the sensor cell120, a porous diffused layer 101, an air duct 102, an insulating layer104, and a heater 103. The gas concentration sensor 100 is installed atthe right side thereof, as viewed in the drawing, on the exhaust pipe 13of the engine so as to expose upper, lower, and left surfaces to exhaustgasses.

The pump cell 110 is disposed on the porous diffused layer 101 so thatit is exposed to the exhaust gasses. A first pump cell electrode 111 ismounted on the upper surface of the pump cell 110. A second pump cellelectrode 112 is mounted on the lower surface of the pump cell 110facing the porous diffused layer 101. The sensor cell 120 is interposedbetween the porous diffused layer 101 and the air duct 102. A firstsensor cell electrode 121 is attached to an upper surface of the sensorcell 120 facing the porous diffused layer 101. A second sensor cellelectrode 122 is attached to a lower surface of the sensor cell 120facing the air duct 102. The exhaust gasses enters the porous diffusedlayer 101 from the left side thereof, as viewed in the drawing, and flowin the right direction.

The pump cell 110 and the sensor cell 120 are each formed with a solidelectrolyte lamination such as an oxygen ion conductive oxide sinteredmember made from ZrO₂, HfO₂, ThO₂, and Bi₂O₃ in which CaO, MgO, Y₂O₃,and Yb₂O₃ are solved as fixing agents. The porous diffused layer 101 ismade of a heat-resisting inorganic matter such as alumina, magnesia,silica, spinel, and mullite.

The first pump cell electrode 111 and the first and second sensor cellelectrodes 121 and 122 are each made of a noble metal with a highcatalytic activity such as platinum (Pt), while the second pumpelectrode 112 is made of a noble metal such as Au—Pt which is inactivewith respect to NOx, that is, hardly decomposes NOx.

The heater 103 is embedded in the insulating layer 104. The insulatinglayer 104 defines the air duct 102 between itself and the sensor cell120. The air duct 102 serves as a reference gas chamber into which theair is introduced. The air in the reference gas chamber is used as areference gas in measuring the concentration of O₂. The insulating layer104 is made of alumina. The heater 103 is made of platinum and cermetsuch as alumina and supplied with power from a heater control circuit,as will be described later in detail, to produce the heat for activatingthe whole of the gas concentration sensor 100.

In operation, when exhaust gasses containing O₂, NOx, CO₂, and H₂O, asshown in FIG. 4(a), enter the porous diffused layer 101 and are passingthe pump cell 110, application of voltage to the pump cell 110 throughthe electrodes 111 and 112 causes the exhaust gasses to undergodecomposition. Since the second pump cell electrode 112 is, as describedabove, made of a noble metal which hardly decomposes NOx, only O₂molecules contained in the exhaust gasses are decomposed or ionized bythe pump cell 110, as shown in FIG. 4(b), which are, in turn, returnedto the exhaust gasses from the first pump cell electrode 111, therebycausing a limiting current (also referred to as a pump cell currentbelow) to flow through the pump cell 110 as a function of theconcentration of O₂ in the exhaust gasses.

The O₂ molecules in the exhaust gasses are usually not decomposed by thepump cell 110 completely, so that residual O₂ molecules reach the sensorcell 120. The application of voltage to the sensor cell 120 causes thefirst sensor cell electrode 121 to decompose the O₂ and NOx molecules,as shown in FIG. 4(c), so that oxygen ions are discharged to the airduct 102 through the second sensor cell electrode 122, thereby causing alimiting current (also referred to as a sensor cell current or a NOxcurrent below) to flow through the sensor cell 120 as a function of theconcentration of NOx.

FIG. 5 shows a V-I relation between the voltage applied to the pump cell110 and the pump cell current (mA) outputted from the pump cell 110.Straight segments of lines extending parallel to the abscissa axisindicate limiting current measurable ranges, respectively, which areshifted to the positive side of voltage applied to the pump cell 110 asthe concentration of O₂ increases. Therefore, if the voltage applied tothe pump cell 110 is kept constant when the concentration of O₂ ischanging, the concentration of O₂ may exceed a corresponding one of thelimiting current measurable ranges, resulting in difficulty in measuringthe concentration of O₂ accurately. This also means that a largequantity of O₂ reaches the sensor cell 120 without being discharged fromthe pump cell 110, thereby causing an error component contained in theNOx current to be increased. In order to avoid this, the voltage to beapplied to the pump cell 110 is regulated so that it changes at a rateequivalent to a rate of change in dc resistance component of the pumpcell 110 as a function of the voltage applied to the pump cell 110.Specifically, the voltage to be applied to the pump cell 110 is changedalong a broken line LX1 so that an output of the pump cell 110 may fallwithin any one of the limiting current measurable ranges at all the timeregardless of the concentration of O₂ in the exhaust gasses.

FIG. 6 shows a V-I relation between the voltage applied to the sensorcell 120 and the sensor cell current (mA) outputted from the sensor cell120. In a range where the concentration of NOx is zero (0) ppm, only acurrent, as indicated by A1, produced by the residual O₂ moleculesflowing through the porous diffused layer 101 to the sensor cell 120 isoutputted from the sensor cell 120 as the offset current. In a rangewhere the concentration of NOx is greater than zero (0) and smaller than1,000 ppm, a current, as indicated by A2, produced by the decompositionof NOx by the sensor cell 120 is also outputted from the sensor cell120. If the voltage applied to the sensor cell 120 exceeds a certainupper limit, it will cause an additional current, as indicated by A3,produced by decomposition of H₂O to be also outputted from the sensorcell 120. Straight segments of lines extending parallel to the abscissaaxis indicate limiting current measurable ranges, respectively, where itis possible to measure the NOx decomposition-produced current and whichare slightly shifted to the positive side of voltage applied to thesensor cell 120 as the concentration of NOx increases. The voltageapplied to the sensor cell 120 is, therefore, controlled along a brokenline LX2 so that an output of the sensor cell 120 may fall within one ofthe limiting current measurable ranges at all the time regardless of theconcentration of NOx in the exhaust gasses.

Returning back to FIG. 1, the gas concentration measuring apparatus alsoincludes an electronic control unit (ECU) 20, a sensor control circuit510, and a heater control circuit 520.

The ECU 20 receives an output of the gas concentration sensor 100 andengine operating data on engine speed, inlet air pressure, watertemperature, and throttle opening measured by known sensors (not shown)to control the quantity of fuel supplied by the fuel injector 12 and theignition timing through an ignition system 15. The ECU 20 also receivesan O₂ concentration signal which is proportional to an air-fuel ratio ofa mixture supplied to the engine 10 and which will also be referred toas an A/F signal and a NOx concentration signal outputted from thesensor control circuit 510.

The sensor control circuit 510 picks up the pump cell current and thesensor cell current from the gas concentration sensor 100 to calculatethe concentrations of O₂ and NOx in the exhaust gasses and outputssignals indicative thereof to the ECU 20. The sensor control circuit 510also picks up data on a sensor element temperature determined as afunction of a sensor element resistance which indicates the active stateof the gas concentration sensor 100 and outputs a signal indicativethereof to the ECU 20.

The heater control circuit 520, as will be described later in detail,receives the sensor element temperature data from the ECU 20 to controlthe power supply to the heater 103 for maintaining the gas concentrationsensor 100 activated.

The sensor control circuit 510 and the heater control circuit 520 arebuilt in a connector 300 connecting between the ECU 20 and the gasconcentration sensor 100. Specifically, in a typical prior artstructure, the sensor control circuit 510 and the heater control circuit520 are disposed in the ECU 20, but in this embodiment, they areintegrated near the gas concentration sensor 100. This is because thereare three main reasons below:

-   -   1 The sensor cell current flowing through the sensor cell is in        the range of 0 to 2.5 μA, corresponding to a NOx concentration        between 0 and 500 ppm. The detection of ±5 ppm requires a        measurement accuracy better than ±25 nA. Such low currents are        very sensitive to electromagnetic fields and noise if flowing in        long wires.    -   2 For the achievement of a resolution of 2.5 nA, the insulation        resistance between wires of a circuit measuring the sensor cell        current. This value is much higher than what can be realized        within standard automotive wiring harness environment.    -   3 The exact calibration for the NOx and O₂ characteristics (gain        and offset) in serial production referring to piece to piece        variation can be more easily achieved with a circuit which is        nearest to the gas concentration sensor 100.

The oxygen concentration determining circuit 511, the NOx concentrationdetermining circuit 512, the impedance measuring circuit 513, and theheater control circuit 520 may be formed on a single bare chip mountedon a ceramic substrate or a ceramic multi-layered board, thereby alsoresulting in a compact structure and greatly improved heat and vibrationresistances. The sensor control circuit 510 connects electrically withthe gas concentration sensor 100 through conductors 401. The heatercontrol circuit 520 connects electrically with the heater 103 through aconductor 402.

The gas concentration sensor 100, as clearly shown in FIG. 11, has acover 160 and a sensor element 150 disposed within the cover 160. Thesensor element 150 consists of the pump cell 110, the sensor cell 120,and the heater 103, as shown in FIG. 3. The cover 160 has formed thereina plurality of pin holes through which the exhaust gasses flow into thecover 160. The connector 300 includes a casing 310 and a plug 320. Thecasing 310 has disposed therein the sensor control circuit 510 and theheater control circuit 520, thereby minimizing addition of externalelectric noises thereto.

The sensor control circuit 510 includes, as clearly shown in FIG. 2, anoxygen concentration determining circuit 511, a NOx concentrationdetermining circuit 512, and a sensor element impedance measuringcircuit 513.

The oxygen concentration determining circuit 511 is connected to thepump cell 110 of the gas concentration sensor 100 to measure an electriccurrent or the pump cell current flowing through the pump cell 110 as afunction of the concentration of O₂ and converts it into a voltagesignal which is, in turn, outputted to the ECU 20. The oxygenconcentration determining circuit 511 is also responsive to the pumpcell current to adjust the voltage applied to the pump cell 110.Similarly, the NOx concentration determining circuit 512 is connected tothe sensor cell 120 to measure an electric current or the sensor cellcurrent flowing through the sensor cell 120 as a function of theconcentration of NOx and converts it into a voltage signal which is, inturn, outputted to the ECU 20. The NOx concentration determining circuit512 is also responsive to the sensor cell current to adjust the voltageapplied to the sensor cell 120.

The sensor element impedance measuring circuit 513 measures theimpedance of the sensor cell 120 or the pump cell 110 in a sweep methodand outputs a signal indicative thereof to the heater control circuit520.

The heater control circuit 520 is responsive to the signal indicative ofthe impedance outputted from the sensor cell impedance measuring circuit513 to control the power supply to the heater 103. Japanese PatentApplication No. 10-275521 and Japanese Patent First Publication No.8-278279 teach heater control systems, disclosure of which isincorporated herein by reference.

The sensor control circuit 510 includes, as clearly shown in FIG. 7, amicrocomputer 200 consisting of a CPU, A/D converters, and D/Aconverters. To the A/D converters A/D0 to A/D3, voltages appearing atterminals Vc, Ve, Vd, and Vb are inputted. From the D/A converters D/A1and D/A0, a pump cell control voltage Vb and the sensor cell controlvoltage Vc are outputted. From the D/A converters D/A2 and D/A3, an O₂concentration output and a NOx concentration output are provided.

Specifically, the pump cell control voltage Vb is inputted to annon-inverting input of the amplifier 211. An output of the amplifier 211is connected to one end of the resistor 212 used in measuring the pumpcell current Ip flowing through the pump cell 110 as a function of theconcentration of O₂. The other end of the resistor 212 is connected tothe first pump cell electrode 111 of the gas concentration sensor 100and an inverting input of the amplifier 211, thereby controlling thevoltage appearing at the first pump cell electrode 111 so as to be keptat the same potential as the pump cell control voltage Vb. The resistor212 also connects at both ends to the A/D converters A/D2 and A/D3.

Therefore, application of the pump cell control voltage Vb to the pumpcell 110 from the sensor control circuit 510 will cause the pump cellcurrent Ip to flow through the resistor 212. The pump cell current Ip isgiven by the following equation:Ip=(Vd−Vb)/R1where Vb and Vd are voltages appearing at the terminals Vb and Vd acrossthe resistor 212, and R1 is a resistance value of the resistor 212.

The microcomputer 200, the amplifier 211, and the resistor 212constitute the oxygen concentration determining circuit 511.

The sensor cell control voltage Vc outputted from the D/A converter D/A0is inputted to an non-inverting input of the amplifier 221 through alow-pass filter 230. The low-pass filter 230 may be a primary filterconsisting of a capacitor. An output of the amplifier 221 is connectedto one end of the resistor 222 used in measuring the sensor cell currentIs flowing through the sensor cell 120 as a function of theconcentration of NOx. The other end of the resistor 222 is connected tothe second sensor cell electrode 122 of the gas concentration sensor 100and an inverting input of the amplifier 221, thereby controlling thevoltage appearing at the second sensor cell electrode 122 to be kept atthe same potential as the sensor cell control voltage Vc. The resistor222 connects at both ends thereof to the A/D converters A/D0 and A/D1 ofthe microcomputer 200.

Therefore, application of the sensor cell control voltage Vc to thesensor cell 120 from the sensor control circuit 510 will cause thesensor cell current Is to flow through the resistor 222. The sensor cellcurrent Is is given by the following equation:Is=(Ve−Vc)/R2where Ve and Vc are voltages appearing at the terminals Ve and Vc acrossthe resistor 222 and R2 is a resistance value of the resistor 222.

The microcomputer 200, the amplifier 221, and the resistor 222constitute the NOx concentration determining circuit 512.

The microcomputer 200 measures an a.c. impedance of the sensor cell 120using the sweep method. Specifically, the measurement of the ACimpedance is achieved by changing the sensor cell control voltage Vcoutputted from the D/A converter D/A0 instantaneously to apply an acvoltage to the sensor cell 120 which is blurred in the form of a sinewave through the low-pass filter 230. The frequency of the ac voltage ispreferably higher than 10 KHz. The time constant of the low-pass filter230 is in the order of 5 μs. The microcomputer 200 monitors changes involtage Ve and Vc appearing at the terminals Ve and Vc through the A/Dconverters A/D1 and A/D0 to determine a change in voltage differenceacross the resistor 222 and a change in sensor current and calculatesthe a.c. impedance of the sensor cell 120 based on the changes involtage difference and sensor current. The microcomputer 200 outputs asignal indicative of the a.c. impedance of the sensor call 120 to theheater control circuit 520 through a D/A converter or a serialcommunication port.

The microcomputer 200, the amplifier 221, and the resistor 222constitute the sensor element impedance measuring circuit 513.

The microcomputer 200 outputs a control signal having a given dutyfactor through an I/O port to operate a MOSFET driver 521. The MOSFETdriver 521 activates the MOSFET 522 to regulate the power supplied froma power source 523 such as a battery to the heater 103 under the PWMcontrol. The microcomputer 200, the MOSFET driver 521, and the MOSFET522 constitute the heater control circuit 520.

FIG. 8 shows a flowchart of a program or a sequence of logical stepsperformed by the CPU of the microcomputer 200 in the course of executionof a main program (not shown), for example, an air-fuel ratio controlprogram to control the pump cell control voltage Vb and the sensor cellcontrol voltage Vc inputted to the pump cell 110 and the sensor cell120.

First, in step 101, the CPU picks up the voltage Vd which is developedat the terminal Vd (i.e., one end of the resistor 212) and convertedinto a digital signal through the A/D converter A/D2. Similarly, insteps 102, 103, and 104, the CPU picks up the voltages Vb, Ve, and Vcwhich are developed at the terminals Vc, Ve, and Vc and converted intodigital signals through the A/D converters A/D3, A/D1, and A/D0,respectively.

After step 104, the routine proceeds to step 105 wherein the pump cellcurrent Ip(=(Vd−Vb)/R1) is determined. The routine proceeds to step 106wherein a target input voltage to be applied to the pump cell 110 isdetermined which corresponds to the pump cell current Ip on the voltageline LX1 shown in FIG. 5. The routine proceeds to step 107 wherein thetarget input voltage determined in step 106 is outputted as the pumpcell control voltage Vb through the D/A converter D/A1.

The routine proceeds to step 108 wherein the sensor cell currentIs(=(Ve−Vc)/R2) is determined. The routine proceeds to step 109 whereina target input voltage to be applied to the sensor cell 120 isdetermined which corresponds to the sensor cell current Is on thevoltage line LX2 shown in FIG. 6. The routine proceeds to step 110wherein the target input voltage determined in step 109 is outputted asthe sensor cell control voltage Vc through the D/A converter D/A0.

The routine proceeds to step 111 wherein the sensor cell current Is isoutputted as indicating the concentration of NOx to the ECU 20 through,for example, a serial communication port. The routine proceeds to step112 wherein the pump cell current Ip is outputted as indicating theconcentration of O₂ to the ECU 20 through, for example, a serialcommunication port.

FIG. 9 shows a subprogram for determining the sensor element impedancewhich is executed by the CPU of the microcomputer 200 selectively atregular intervals of 128 ms in a start-up mode of engine operation andat regular intervals of 256 ms after the engine is warmed up.

After entering the program, the routine proceeds to steps 201 and 202wherein the voltages Ve and Vc developed across the resistor 222 arepicked up through the A/D converters A/D1 and A/D0, which will bereferred to as Ve1 and Ve2 below.

The routine proceeds to step 203 wherein the sum of a sensor cellcontrol voltage Vs now applied to the sensor cell 120 and an additionala.c. voltage ΔVs is outputted from the D/A converter D/A0, therebycausing, as shown in FIG. 10, the voltages Vc and Ve developed acrossthe resistor 222 to change in the form of a sine wave according to thetime constant of the low-pass filter 230.

The routine proceeds to steps 204 and 205 wherein the voltages appearingat the terminals Ve and Vc, which will be referred to as Ve2 and Vc2below, are picked up 20 μs after the voltage applied to the resistor 222is changed in step 203.

The routine proceeds to step 206 wherein the impedance Zac of the sensorcell 120 is calculated according to an equation below:Zac=(Vc2−Vc1)/{(Ve2−Vc2)−(Ve1−Vc1)}

The routine proceeds to step 207 wherein a negative voltage Δ Vs2 is, asshown in FIG. 10, outputted from the D/A converter D/A0 temporarily toreturn the voltage applied to the sensor cell 120 to the voltage Vs.

An electric current flowing through the gas concentration sensor 100 asa function of the concentration of each of O₂ and NOx is, as describedabove, extremely weak, so that it apt to interfere with electricalnoises produced from peripheral devices. Particularly, when theconcentration of NOx is within 0 to 2000 ppm, the current outputted fromthe gas concentration sensor 100 as a function of the concentration ofNOx is, as shown in FIG. 6, as little as 5 to 10 μA, thus resulting in afailure in measuring the concentration of NOx accurately. In order toavoid this problem, this embodiment specifies the length of theconductors 401 connecting between the gas concentration sensor 100 andthe sensor control circuit 510 and the length of the conductor 402connecting between the heater 103 and the heater control circuit 520using a suitable relation, as shown in FIG. 12, between the length of awire extending from each of a cup-shaped A/F sensor, a laminated A/Fsensor, and a NOx sensor (i.e., the gas concentration sensor 100) andthe level of an output signal thereof.

Generally, a gas concentration sensor such as the one in this embodimentdesigned to measure the concentration of NOx is required to shorten thelength of wire extending therefrom as compared with the cup-shaped orlaminated A/F sensors. Minimizing the interference of an output of thegas concentration sensor 100 with electric noises, thus, requiresdecreasing the length of the conductors 401 and 402. Further, in a casewhere the gas concentration sensor 100 is mounted in an automotivevehicle, various electric noises are added to an output of the gasconcentration sensor 100. Therefore, the lower the level of the outputof the gas concentration sensor 100, the better the decrease in distancebetween the gas concentration sensor 100 and the connector 300 tominimize the electric noises.

The sensor element of the gas concentration sensor 100 contains ceramic,so that it has characteristics of a flow of d.c. current, and a flow ofa.c. current, and an output indicating the concentration of gas thatundergo an inevitable unit-to-unit deviation in mass production, thusresulting in a decrease in production yield. Specifically, a smallvariation in production condition will cause the characteristics and theimpedance of sensors to change. Some of the sensors whosecharacteristics are below standards are usually discarded, thusresulting in a decrease in production yield. For example, when theconcentration of O₂ varies, as shown in FIG. 13, an actual output of thegas concentration sensor 100, as indicated by a solid line, is shiftedfrom a correct one, as indicated by a broken line.

Additionally, the resistance value of the heater 103 is set small inorder to speed up the activity of the gas concentration sensor 100. Atthe start of the control of the heater 103, it is usually difficult tomeasure the impedance of the gas concentration sensor 100 sensoraccurately. The ECU 20 may, thus, monitor the power supplied to theheater 103 from the heater control circuit 520 (i.e., the heater voltageand current) and provide a power supply control signal to the heatercontrol circuit 520. If, therefore, the resistance value of theconductor 402 (including resistance values of the heater 103 and theheater control circuit 520) differs among vehicles, it will cause thecontrollability of the heater 103 to vary, resulting in, for example, adecrease in heat produced by the heater 103 and an error in measuringthe power supplied to the heater 103, which may lead to a delay inactivating the gas concentration sensor 100 and overheating thereof.

In order to avoid the above problems, the sensor control circuit 510 ofthis embodiment is, as described later in detail, designed to adjust orcorrect the characteristics of the gas concentration sensor 100, and theheater control circuit 520 is designed to compensate for the error inmeasuring the power supplied to the heater 103 depending upon theresistance of the conductor 402.

Specifically, the correction of the characteristics of the gasconcentration sensor 100 and the compensation of the error in measuringthe power supplied to the heater 103 are accomplished with gainadjustment and offset adjustment in the sensor control circuit 510 andthe heater control circuit 520 in manufacturing processes. Suchadjustments may be achieved with

-   -   1 installation of adjustment parts,    -   2 installation and trimming of a thick-film resistor, or    -   3 trimming a resistor on an IC chip in which the sensor control        circuit 510 and the heater control circuit 520 are integrated.

As one example, correction circuit 531, 532, and 533, as shown in FIG.14, may be connected to outputs of the oxygen concentration determiningcircuit 512, the NOx concentration determining circuit 512, and theimpedance measuring circuit 513, respectively. Each of the correctioncircuits 531, 532, and 533 is made of a resistor such as a shunt whichis trimmed to adjust a resistance value thereof so as to bring an actualoutput of a corresponding one of the oxygen concentration determiningcircuit 511, the NOx concentration determining circuit 512, and theimpedance measuring circuit 533 into agreement with a correct or desiredone.

Alternatively, a gain/offset adjustment map may be pre-stored in themicrocomputer 200 which is used in calculating and adjusting a gain oran offset of the amplifiers 211 and 221 to bring an actual output ofeach of the oxygen concentration determining circuit 512, the NOxconcentration determining circuit 512, and the impedance measuringcircuit 533 into agreement with a correct or desired one. Instead of useof the gain/offset adjustment map, parameters used in calculating andadjusting the gain and offset may be inputted directly to themicrocomputer 200 through an A/D converter.

In order to eliminate an error component contained in an output signalof the heater control circuit 520 indicating the amount of powersupplied to the heater 103 due to the resistance of the conductor 402, acorrection circuit 534, similar to one of the correction circuits 531,532, and 533, may be built in the heater control circuit 520.Alternatively, the microcomputer 200 may calculate a gain or an offsetof the heater control circuit 520 in the same manner as described aboveto bring the output signal into agreement with a correct one.

While the present invention has been disclosed in terms of the preferredembodiments in order to facilitate better understanding thereof, itshould be appreciated that the invention can be embodied in various wayswithout departing from the principle of the invention. Therefore, theinvention should be understood to include all possible embodiments andmodifications to the shown embodiments which can be embodied withoutdeparting from the principle of the invention as set forth in theappended claims.

For example, only the NOx concentration determining circuit 512 whichreceives a weak electrical signal (i.e., the sensor cell current) fromthe gas concentration sensor 100 may be disposed within the connector300 to shorten the distance to the gas concentration sensor 100 or thelength of a conductor connecting between the NOx concentrationdetermining circuit 512 and the gas concentration sensor 100 forminimizing addition of electrical noises. Additionally, any one of theoxygen concentration determining circuit 511, the impedance measuringcircuit 513, and the heater control circuit 520 may also be disposedwithin the connector 300, thereby allowing the length of conductorsbetween the circuits 511, 512, 513, and 520 and the gas concentrationsensor 100 to be determined selectively, thus resulting in an increasein freedom of design.

The first sensor cell electrode 121 and the second pump cell electrode112 are, as clearly shown in FIG. 7, connected to ground, but mayalternatively be connected to a common terminal which is kept at apositive potential. This allows a negative electric current to flowthrough each of the pump cell 110 and the sensor cell 120. Thus, evenwhen a rich gas which usually reduces a flow of the negative current andchanges a balance of concentration of O₂ in the porous diffused layer101 enters the gas concentration sensor 100, it becomes possible to keepthe concentration of gas, for example, O₂ in the porous diffused layer101 at a constant value equivalent to the stoichiometric. This enablesthe rich gas to be measured accurately, thus resulting in an increase inmeasurable range of the gas concentration sensor 100 and also results ingreatly improved response rate of the gas concentration sensor 100 whenthe gas returns from the rich to lean side.

The present invention may be used with an air-fuel ratio (A/F) sensordesigned to measure the concentration of O₂ contained in exhaust gassesof an internal combustion engine for determining an air-fuel ratio of amixture supplied to the engine. As such an A/F sensor, a cup-shaped A/Fsensor in which a solid electrolyte body and a diffused resistance layerare cup-shaped and a laminated A/F sensor made of a lamination of asolid electrolyte plate and a diffused resistance layer are known. Whenthe air-fuel ratio is 12 to 18, the laminated A/F sensor outputs acurrent signal of as little as −0.75 to 0.4 mA, but the structure ofthis invention reduces addition of electric noises to an output of thelaminated A/F sensor sufficiently.

The present invention may also be used with three-cell or four-cell gasconcentration sensors which are known in the art.

The present invention may further be used with a gas concentrationsensor which is designed to decompose and discharge O₂ contained ingasses to be measured through a pump cell and decompose HC and/or COcontained in the gasses after the decomposition of O₂ through a sensorcell for determining the concentration of O₂ and the concentration of HCand/or CO.

1. A gas concentration measuring apparatus comprising: a gasconcentration sensor having a sensor element and an electrical connectorfor connection to a remote digital signal processor, said sensor elementincluding a pump cell and a sensor cell, the pump cell being made of asolid electrolyte body and a first and a second pump cell electrode, thefirst and second pump cell electrodes being responsive to application ofvoltage to disassociate and pump oxygen molecules contained in exhaustgasses of an automotive engine to which said gas concentration sensor isexposed out of said gas concentration sensor, said sensor cell beingmade of a solid electrolyte body and a first and a second sensor cellelectrode, the first and second sensor cell electrodes being responsiveto application of voltage to disassociate at least one of NO_(x), HC,and CO contained in the exhaust gases through the first sensor cellelectrode to produce a current signal flowing through the solidelectrolyte body as a function of concentration of the at least one ofNO_(x), HC, and CO; and a microcomputer disposed within said connectorperforming functions of a gas concentration determining and heatercontrol, the gas concentration determining being functionally connectedto the first and second sensor cell electrodes to process and analyzethe current signal provided by said gas concentration sensor to outputdata as a function of the concentration of the at least one of NO_(x),HC, and CO to said remote digital signal processor through serialdigital signal communication, the heater control function controllingpower supply to a heater.
 2. A gas concentration measuring apparatus asin claim 1 wherein said gas concentration sensor has an expected minimumlevel of output current during normal sensing operation and furthercomprising: a conductor electrically connecting said gas concentrationsensor and said microcomputer for transmission of the current signalfrom said gas concentration sensor to said microcomputer, said conductorhaving a length selected as a function of said expected minimum level ofthe current signal outputted from said gas concentration sensor.
 3. Agas concentration measuring apparatus as in claim 2 wherein saidmicrocomputer has a function of compensating for a unit-to-unitvariation in characteristic of said gas concentration sensor.
 4. A gasconcentration measuring apparatus as in claim 1 wherein saidmicrocomputer has a function of compensating for a unit-to-unitvariation in characteristic of said gas concentration sensor.
 5. A gasconcentration measuring apparatus comprising: a gas concentration sensorhaving a sensor element and an electrical connector for connection to aremote digital signal processor, said sensor element including a pumpcell and a sensor cell, the pump cell being made of a solid electrolytebody and a first and a second pump cell electrode, the first an secondpump cell electrodes being responsive to application of voltage todisassociate and pump oxygen molecules contained in exhaust gasses of anautomotive engine to which said gas concentration sensor is exposed outof said gas concentration sensor, said sensor cell being made of a solidelectrolyte body and a first and a second sensor cell electrode, thefirst and second sensor cell electrodes being responsive to applicationof voltage to disassociate at least one of NO_(x), HC, and CO containedin the exhaust gasses through the first sensor cell electrode to producea current signal flowing through the solid electrolyte body as afunction of concentration of the at least one of NO_(x), HC, and CO; anda microcomputer disposed within said connector including a gasconcentration determining circuit and a heater control circuit, the gasconcentration determining circuit being connected to the first andsecond sensor cell electrodes and processing the current signal providedby said gas concentration sensor to output a voltage signal as afunction of the concentration of the at least one of NO_(x), HC, and COto said remote digital signal processor through serial digital signalcommunication, the heater control circuit controlling power supply to aheater.
 6. A gas concentration measuring apparatus as in claim 5 whereinsaid gas concentration sensor has an expected minimum level of outputcurrent during normal sensing operation and further comprising: aconductor electrically connecting said gas concentration sensor and saidmicrocomputer for transmission of the current signal from said gasconcentration sensor to said microcomputer, said conductor having alength selected as a function of said expected minimum level of thecurrent signal outputted from said gas concentration sensor.
 7. A gasconcentration measuring apparatus as in claim 5 wherein saidmicrocomputer has a function of compensating for a unit-to-unitvariation in characteristic of said gas concentration sensor.
 8. Amethod for operating a gas concentration sensor having a sensor elementand an electrical connector for connection to a remote digital signalprocessor, said sensor element including a pump cell and a sensor cell,the pump cell being made of a solid electrolyte body and a first and asecond pump cell electrode, the first and second pump cell electrodesbeing responsive to application of voltage to disassociate and pumpoxygen molecules contained in exhaust gasses of an automotive engine towhich said gas concentration sensor is exposed out of said gasconcentration sensor, said sensor cell being made of a solid electrolytebody and a first and a second sensor cell electrode, the first andsecond sensor cell electrodes being responsive to application of voltageto disassociate at least one of NO_(x), HC, and CO contained in theexhaust gasses through the first sensor cell electrode to produce acurrent signal flowing through the solid electrolyte body as a functionof concentration of the at least one of NO_(x), HC, and CO, said methodcomprising: providing in said connector a microcomputer performingfunctions of a gas concentration determining and heater control, the gasconcentration determining being functionally connected to the first andsecond sensor cell electrodes to process and analyze the current signalprovided by said gas concentration sensor to output data as a functionof the concentration of the at least one of NO_(x), HC, and CO to saidremote digital signal processor through serial digital signalcommunication, the heater control function controlling power supply to aheater.
 9. A method as in claim 8 wherein said gas concentration sensorhas an expected minimum level of output current during normal sensingoperation and wherein a gas concentration measuring apparatus as aconductor electrically connects said gas concentration sensor and saidmicrocomputer for transmission of the current signal from saidconcentration sensor to said microcomputer, said conductor having alength selected as a function of said expected minimum level of thecurrent signal outputted from said gas concentration sensor.
 10. Amethod as in claim 9 wherein said microcomputer has a function ofcompensating for a unit-to-unit variation in characteristic of said gasconcentration sensor.
 11. A method as in claim 8 wherein saidmicrocomputer has a function of compensating for a unit-to-unitvariation in characteristic of said gas concentration sensor.
 12. Amethod for operating a gas concentration sensor having a sensor elementand an electrical connector for connection to a remote digital signalprocessor, said sensor element including a pump cell and a sensor cell,the pump cell being made of a solid electrolyte body and a first and asecond pump cell electrode, the first and second pump cell electrodesbeing responsive to application of voltage to disassociate and pumpoxygen molecules contained in exhaust gasses of an automotive engine towhich said gas concentration sensor is exposed out of said gasconcentration sensor, said sensor cell being made of a solid electrolytebody and a first and a second sensor cell electrode, the first andsecond sensor cell electrodes being responsive to application of voltageto disassociate at least one of NO_(x), HC, and CO contained in theexhaust gasses through the first sensor cell electrode to produce acurrent signal flowing through the solid electrolyte body as a functionof concentration of the at least one of NO_(x), HC, and Co, said methodcomprising: providing in said connector a microcomputer including a gasconcentration determining circuit and a heater control circuit, the gasconcentration determining circuit being connected to the first andsecond sensor cell electrodes and processing the current signal providedby said gas concentration sensor to output a voltage signal as afunction of the concentration of the at least one of NO_(x), HC, and COto said remote digital signal processor through serial digital signalcommunication, the heater control circuit controlling power supply to aheater.
 13. A method as in claim 12 wherein said gas concentrationsensor has an expected minimum level of output current during normalsensing operation and wherein a gas concentration measuring apparatus asa conductor electrically connects said gas concentration sensor and saidmicrocomputer for transmission of the current signal from saidconcentration sensor to said microcomputer, said conductor having alength selected as a function of said expected minimum level of thecurrent signal outputted from said gas concentration sensor.
 14. Amethod as in claim 12 wherein said microcomputer has a function ofcompensating for a unit-to-unit variation in characteristic of said gasconcentration sensor.
 15. A gas concentration measuring apparatuscomprising: a gas concentration sensor having a sensor element and anelectrical connector for connection to a remote digital signalprocessor, said sensor element including a pump cell and a sensor cell,the pump cell being made of a solid electrolyte body and a first and asecond pump cell electrode, the first and second pump cell electrodesbeing responsive to application of voltage to disassociate and pumpoxygen molecules contained in exhaust gases of an automotive engine towhich said gas concentration sensor is exposed out of said gasconcentration sensor, said sensor cell being made of a solid electrolytebody and a first and a second sensor cell electrode, the first andsecond sensor cell electrodes being responsive to application of voltageto disassociate at least one of NO_(x), HC, and CO contained in theexhaust gases through the first sensor cell electrode to produce acurrent signal flowing through the solid electrolyte body as a functionof concentration of the at least one of NO_(x), HC, and CO; and amicrocomputer disposed within said connector performing functions of agas concentration determining, impedance measuring, and heater control,the gas concentration determining being functionally connected to thefirst and second sensor cell electrodes to process and analyze thecurrent signal provided by said gas concentration sensor to output dataas a function of the concentration of the at least one of HO_(x), HC,and CO to said remote digital signal processor through serial digitalsignal communication, the impedance measuring function measuring animpedance of the sensor element of said gas concentration sensor, theheater control function controlling power supply to a heater which heatsthe sensor element based on the measured impedance; wherein saidmicrocomputer compensates for unit-to-unit variation in characteristicsof said gas concentration sensor.
 16. A gas concentration measuringapparatus as in claim 15 wherein said microcomputer measures a currentflowing through the first and second pump cell electrodes of said pumpcell and determines a target voltage to be applied to the first andsecond pump cell electrodes as a function of the measured current.
 17. Agas concentration measuring apparatus as in claim 15 wherein said gasconcentration sensor has an expected minimum level of output currentduring normal sensing operation and further comprising: a conductorelectrically connecting said gas concentration sensor and saidmicrocomputer for transmission of the current signal from said gasconcentration sensor to said microcomputer, said conductor having alength selected as a function of said expected minimum level of thecurrent signal outputted from said gas concentration sensor.
 18. Amethod for operating a gas concentration sensor having a sensor elementand an electrical connector for connection to a remote digital signalprocessor, said sensor element including a pump cell and a sensor cell,the pump cell being made of a solid electrolyte body and a first and asecond pump cell electrode, the first and second pump cell electrodesbeing responsive to application of voltage to disassociate and pumpoxygen molecules contained in exhaust gases of an automotive engine towhich said gas concentration sensor is exposed out of said concentrationsensor, said sensor cell being made of a solid electrolyte body and afirst and a second sensor cell electrode, the first and second sensorcell electrodes being responsive to application of voltage todisassociate at least one of NO_(x), HC, and CO contained in the exhaustgases through the first sensor cell electrode to produce a currentsignal flowing through the solid electrolyte body as a function ofconcentration of the at least one of NO_(x), HC, and CO, said methodcomprising: providing in said connector a microcomputer performingfunctions of gas concentration determining, impedance measuring, andheater control, the gas concentration determining being functionallyconnected to the first and second sensor cell electrodes to process andanalyze the current signal provided by said gas concentration sensor tooutput data as a function of the concentration of the at least one ofHO_(x), HC, and CO to said remote digital signal processor throughserial digital signal communication, the impedance measuring functionmeasuring an impedance of the sensor element of said gas concentrationsensor, the heater control function controlling a power supply to aheater which heats the sensor element based on the measured impedance;wherein said microcomputer compensates for unit-to-unit variation incharacteristics of said gas concentration sensor.
 19. A method as inclaim 18 wherein said microcomputer measures a current flowing throughthe first and second pump cell electrodes of said pump cell anddetermines a target voltage to be applied to the first and second pumpcell electrodes as a function of the measured current.
 20. A method asin claim 18 wherein said gas concentration sensor has an expectedminimum level of output current during normal sensing operation andwherein a gas concentration measuring apparatus as a conductorelectrically connects said gas concentration sensor and saidmicrocomputer for transmission of the current signal from said gasconcentration sensor to said microcomputer, said conductor having alength selected as a function of said expected minimum level of thecurrent signal outputted from said gas concentration sensor.